Thus, the DNA for a cell must be packaged in a very ordered way to fit and function within a structure (the cell) that is not visible to the naked eye. Stretched end-to-end, the DNA molecules in a single human cell would come to a length of about 2 meters. Contrary to your claim, DNA strands do have sufficient periodicity to produce an x-ray diffraction pattern: in essence, when seen from the side, the helix collapses to two sets of diagonal lines, each of which acts as a diffraction grating, producing as an end result a cross formed from two diagonal sets of dots. In addition, DNA molecules can be very long. For this reason, the DNA is protected and packaged in very specific ways. These include messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA)-molecules that are involved in the production of proteins from the DNA code.ĭNA is a working molecule it must be replicated when a cell is ready to divide, and it must be “read” to produce the molecules, such as proteins, to carry out the functions of the cell. The double helical structure described by Watson and Crick was also B-DNA. Molecular biologists have named several kinds of RNA on the basis of their function. It was based primarily on model building and X-ray diffraction pattern. However, they do not contain thymine, which is instead replaced by uracil, symbolized by a “U.” RNA exists as a single-stranded molecule rather than a double-stranded helix. The theory of X-ray diffraction from ideal, rigid helices allowed Watson and Crick to unravel the DNA structure, thereby elucidating functions encoded in it. RNA nucleotides contain the nitrogenous bases adenine, cytosine, and guanine. The A form occurs under non-physiological conditions in partly. Compared to B-DNA, the A-DNA form is a wider right-handed spiral, with a shallow, wide minor groove and a narrower, deeper major groove. \): The difference between the ribose found in RNA and the deoxyribose found in DNA is that ribose has a hydroxyl group at the 2' carbon. Their corresponding X-ray diffraction and scattering patterns are characteristic of molecular paracrystals with a significant degree of disorder.
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